This invention relates to electrical machines.
FIGS. 1a and 1b shows a conventional two-phase variable reluctance motor comprising a stator 2 having two pairs 3, 4 of oppositely disposed inwardly directed salient poles provided with two pairs 5, 6 of energising windings corresponding to the two phases, and a rotor 7 having a single pair 8 of oppositely disposed outwardly directed salient poles without windings. Each of the four energising windings is wound about its corresponding pole, as indicated by the symbols Yxe2x80x94Y denoting two diametrically opposite portions of each winding of the winding pair 6 and the symbols Xxe2x80x94X denoting two diametrically opposite portions of each winding of the winding pair 5. An excitation circuit (not shown) is provided for rotating the rotor 7 within the stator 2 by alternately energising the stator windings in synchronism with rotation of the rotor so that torque is developed by the tendency of the rotor 7 to arrange itself in a position of minimum reluctance within the magnetic field produced by the windings, as will be described in more detail below. Such a variable reluctance motor offers the advantage over a conventional wound rotor motor that a commutator and brushes, which are wearing parts, are not required for supply of current to the rotor. Furthermore other advantages are provided because there are no conductors on the rotor and high-cost permanent magnets are not required.
The symbols + and xe2x88x92 in FIGS. 1a and 1b show the directions of current flow in the windings in the two alternate modes of excitation in which the rotor 7 is attracted either to the horizontal position or to the vertical position as viewed in the figures. It will be appreciated that rotation of the rotor 7 requires alternate energisation of the winding pairs 5 and 6, preferably with only one winding pair 5 or 6 being energised at a time, and with the current usually being supplied to each winding pair 5 or 6 in only one direction during such energisation. However the windings can only be energised for a maximum of half the time per revolution if useful torque is to be produced, so that highly efficient utilisation of the electric circuit is not possible with such a motor.
By contrast a fully pitched two-phase variable reluctance motor, as described by J. D. Wale and C. Pollock, xe2x80x9cNovel Converter Topologies for a Two-Phase Switched Reluctance Motor with Fully Pitched Windingsxe2x80x9d, IEEE Power Electronics Specialists Conference, Braveno, June 1996, pp. 1798-1803 and as shown in FIGS. 2a and 2b (in which the same reference numerals are used to denote like parts as in FIGS. 1a and 1b), comprises two windings 10 and 11 having a pitch which is twice the pole pitch of the motor, that is 180xc2x0 in the example illustrated, and disposed at 90xc2x0 to one another. The winding 11 may be wound so that one part of the winding on one side of the rotor 7 fills a stator slot 12 defined between adjacent poles of the pole pairs 3, 4, and another part of the winding 11 on the diametrically opposite side of the rotor 7 fills a stator slot 13 defined between two further adjacent poles of the pole pairs 3, 4. The winding 10 has corresponding parts filling diametrically opposed stator slots 14 and 15. Thus the two windings 10 and 11 span the width of the motor with the axes of the windings 10, 11 being at right angles to one another.
Furthermore two alternate modes of excitation of such a motor corresponding to the horizontal and vertical positions of the rotor 7 are shown in FIGS. 2a and 2b from which it will be appreciated that both windings 10, 11 are energised in both modes of excitation, but that, whereas the direction of current flow in the winding 10 is the same in both modes, the direction of current flow in the winding 11 changes between the two modes. Since current is supplied to both phase windings 10, 11 in both modes and since each winding 10 or 11 occupies half the total stator slot area, such a system can achieve 100% utilisation of its slot area. This contrasts with the 50% utilisation achieved with the conventional wound variable reluctance motor described above in which only one phase winding is energised at a time. Furthermore, since there is no requirement for the direction of current in the winding 10 to change, the winding 10, which may be termed the field winding, can be supplied with direct current without any switching which leads to simplification of the excitation circuit used. However the winding 11, which may be termed the armature winding, must be energised with current which alternates in synchronism with the rotor position so as to determine the changing orientation of the stator flux required to attract the rotor alternately to the horizontal and vertical positions. Such a motor may be termed xe2x80x9ca flux-switching motorxe2x80x9d. The need to supply the armature winding with alternating current in such a motor can result in an excitation circuit of high complexity and cost.
J. R. Surano and C-M Ong, xe2x80x9cVariable Reluctance Motor Structures for Low-Speed Operationxe2x80x9d, IEEE Transactions on Industry Applications, Vol.32, No.2, March/April 1996, pp 808-815 and UK Patent No. 2262843 also disclose fully pitched two-phase variable reluctance motors. The motor disclosed in UK Patent No. 2262843 is a three-phase variable reluctance motor having three windings which must be energised with current in synchronism with rotation of the rotor so that such a motor requires an excitation circuit of high complexity.
WO 98/05112 discloses a fully pitched two-phase variable reluctance motor having a four-pole stator 2 which, as shown diagranmatically in FIG. 3, is provided with a field winding 10 and an armature winding 11 each of which is split into two coils 22 and 23 or 24 and 25 closely coupled and wound so that diametrically opposite portions of both coils are disposed within diametrically opposite stator slots. FIG. 4 shows a generalised circuit diagram for energising the armature coils 24 and 25. The coils 24 and 25 are connected within the circuit so that direct current supply to the terminals 26 and 27 flows through both coils 24 and 25 in the same direction so as to generate magnetomotive forces in opposite direction as a result of the opposite winding of the coils. Switches 28 and 29, which may comprise field effect transistors or thyristors for example, are connected in series with the coils 24 and 25 and are switched alternately to effect alternate energisation of the coils 24 and 25 so as to provide the required magnetomotive forces acting in opposite directions. It is an advantage of such an arrangement that the armature winding is made up of two closely coupled coils which enables each coil to be energised with current in only one direction so that relatively simple excitation circuitry can be used.
GB 18027 dated Sep. 9, 1901 discloses a variable reluctance machine having sets of windings on the stator which are alternately energised so as to provide the required interaction with the rotor. Furthermore GB 554827 discloses an inductor alternator in which the relative arrangement of the stator and rotor teeth produces successive zones of relatively high and low reluctance, and in which field and alternative current windings are provided on the stator to effect the required energisation. However, neither of these prior arrangements possesses the advantageous feature of the arrangement of WO 98/05112.
It is an object of the invention to provide an electrical machine exhibiting high power efficiency which can be produced at relatively low cost.
According to the present invention, there is provided an electrical machine comprising a rotor without windings, a stator having armature windings comprising at least two coils having active portions positioned within armature winding slots in a stator iron, and field windings having active portions positioned within field winding slots in the stator iron so as to generate magnetomotive forces in directions extending transversely of the magnetomotive forces generated by the armature windings, and circuit means for controlling the currents in the coils in synchronism with rotation of the rotor such that periods in which a magnetomotive force is generated in one direction by current flow in one of the coils alternate with periods in which a magnetomotive force is generated in the opposite direction by current flow in another of the coils, characterised in that the armature winding slots and the field winding slots are equal in number and alternate with one another in the stator iron, and in that, considering the width of each slot as being the maximum extent of the slot in the direction of rotation of the rotor and considering the depth of each slot as being the maximum extent of the slot radially of the rotor and the thickness of the back iron behind the slot as being the distance between the maximum extent of the slot and the maximum extent of the armature iron along the same radial direction, the width of each armature winding slot is greater than the width of each field winding slot and the thickness of the back iron behind each armature winding slot is greater than the thickness of the back iron behind each field winding slot.
Such an arrangement allows the slots for the armature windings and the field windings to be adapted to the particular requirements of the windings so as to optimise the power efficiency whilst making best use of magnetic material. Typically, where the stator has a circular cross-section, the field windings need to carry only DC current so that their self-inductance is relatively unimportant and thus the field winding slots can be relatively narrow and deep without compromising the performance. On the other hand, it is desirable for the armature winding slots to be relatively wide and shallow to reduce the self-inductance. This ensures a relatively thick back iron to the stator windings in order to limit iron losses. The relatively thinner back iron behind the field windings does not generate high iron losses in view of the substantially constant level of the magnetic flux associated with the field windings.